Scientific classification is a means used by biologists to organize each of the organisms on the planet, based primarily on evolutionary similarity as determined by visual observation. This system was devised by Carolus Linnaeus (1707-1778).

The system works by placing each organism into a layered hierarchy of groups. Each group at a given layer is composed of a set of groups from the layer directly below. Therefore, in theory, one needs know only the lowest layer (species) of a particular organism in order to uniquely determine the other six layers. In practice, however, many species actually have the same species designation, so when specifiying a species, scientists use the bottom two layers - a system called binomial nomenclature.

The standard groupings (taxa) of taxonomy from most general to most specific are:

Several acronym mnemonics have been made for these, for instance King Phillip called out for good soup. Sometimes tribes, which lie between families and genera, and races, which lie below species, are also used. Intermediate ranks may be created by adding prefixes, for instance:

Superorder

Order

Suborder

Infraorder

The term varieties is sometimes used in place of subspecies. In horticulture, it refers to populations modified by selective breeding, for instance the Peace Rose (http://www.corecom.net/~gardener/Roses/PeaceRose), a hybrid Tea Rose. At the top of the scale, there has been a move towards the three domain system. The domains originally were replacements for the different kingdoms, but often count as a higher rank.

Could add a description of the difficulty in classifying microbes: their features are derived from direct visual observation, but include such procedural characteristics as Gram stain type, motility, ability to form spores, etc. However, given an unknown bacterium with a given set of characteristics, it is in general not possible to predict its phylogeny, toxicity, etc. Other methods, using genes, their DNA, and several types of RNA, are under development.